Relating to communication that uses optical or electronic signals, level control by amplification or attenuation of a signal is known. Relating to this level control, automatic gain control unit (Automatic Gain Control: AGC) in which control is performed such that the level ratio (gain) of the signal light before and after amplification becomes constant, and automatic level control unit (ALC: Automatic Level Control) in which control is performed such that the level of the signal light after amplification becomes constant, are known (Japanese Laid-open Patent Publication No. 2006-230020). Also, a process by which the distortion property of an amplifier is measured and the distortion property of the amplifier is compensated for is known (Japanese Laid-open Patent Publication No. 2003-513498). Also, maintaining the electrical output level of a light receiving device constant in the case that the wavelength of the optical signal has changed is known (Japanese Laid-open Patent Publication No. 2008-53869).
Now, according to a communication system using a transfer path, due to vibrations or impacts to the transfer path, or tension applied thereto, or by variations in temperature or humidity occurring, the polarization of the signal flowing through the transfer path is rotated, and variations to signal strength occur. For example, with a communication system performing information transfer with a high-frequency signal, deterioration of receiving property by such signal variations has to be prevented. Specifically, vibrations, impacts, and tension load are suppressed, optical power variations due to changes in temperature and humidity are suppressed, and deterioration in receiving properties is prevented, by preparing a dedicating space to install a transfer path underground where environment variations are few, and by installing the transfer path in this space. Also, various standards are set for the devices making up the communication system, thereby improving reliability.
However, there are limits to the reliability gained by such measures. For the given problems, there is neither disclosure nor suggestion in Japanese Laid-open Patent Publication No. 2006-230020, Japanese Laid-open Patent Publication No. 2003-513498, or Japanese Laid-open Patent Publication No. 2008-53869 and neither disclosure nor suggestions for the configuration or the like to solve these problems.